DC to DC Converter (Switched Mode Power Supply) Design
Why use dc/dc converters?
In many designs there arises the need to convert one voltage to another. Linear regulators offer a simple low cost solution, but the heat they generate is often inconvenient due to the bulky heatsinking needed to dissipate this heat.
A more elegant solution is a dc/dc converter (switched mode power supply).
Many application notes and datasheets assume that the reader has a basic understanding of switched mode power supplies which can leave the circuit designer with a circuit that works, but he is unsure of exactly how.
This article takes the reader through the basics of dc/dc converter design and shows that switched mode power supply design can be designed with the minimum of mathematical skills. All examples are backed up with LTspice® simulations.
All of the architectures below rely on a controller IC to switch a voltage across an inductor causing the inductor to store charge. This charge is then released into the output capacitor to generate the appropriate output voltage. To understand how they work, we must first understand how an inductor works.
Inductors
When a voltage is switched across an inductor, the current through the inductor behaves according to the equation:
where V is the voltage across the inductor (in Volts), L is the inductance value (in Henries), di is the change in current (in Amps) and dt is the change in time (in seconds). Thus if a fixed voltage is applied to an inductor (of fixed value) this generates a current through the inductor that ramps up linearly with time – a constant change in current with time.
When the voltage is removed from the inductor, the inductor generates its own voltage to try to maintain the current flow. Harnessing this voltage is what enables dc/dc converters to generate higher and lower voltages from the input supply.
Any of the following design guides will explain the operation of the inductor in more detail.
There are 4 basic types of non isolated dc/dc converter:
Buck Converter Design
These convert a high voltage to a lower voltage, mostly converting a positive high voltage to a positive lower voltage.
Boost Converter Design
These convert a low voltage to a higher voltage, mostly converting a positive low voltage to a positive higher voltage.
Buck Boost Converter Design
These produce a fixed output voltage when the input voltage is either higher or lower than the output voltage.
Inverting Converter Design
These produce a negative voltage from a positive voltage. Many text books call this type of converter a buck boost converter, however this nomenclature is normally exclusively reserved for text books. Most practicing engineers refer to a buck boost converter as defined above.
There are also several different types of isolated converter, but the most popular one for low power is the flyback converter:
Flyback Converter Design
These use a transformer, but driven in such a way that the primary and secondary windings appear as 2 separate inductors to the surrounding electronics. Therefore, in architecture flyback converters are very similar to boost converters.
Current Mode dc/dc Converter Operation
Explains how a current mode dc/dc converter operates with respect to the error amplifier, the feedback network, the current sense resistor and the soft start pin
Feedback Resistor Calculator
The feedback resistor values can be calculated using this spreadsheet:
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